This invention is directed to a heat treatable, high hardenability die steel characterized by the ability to achieve a lustrous polished surface of exceptional flatness and smoothness. Additionally, the steel of this invention exhibits an unusual combination of high hardness and high toughness, properties which are generally not complementary. More particularly, this invention is directed to a mold steel for use in the plastic molding art.
The growth of the plastic industry has brought about a strong demand for new and better mold steels. This demand to develop better mold steels has resulted from higher pressures being used in compression molding, the abrasiveness of thermal setting resins such as the phenolics, and the need for better dimensional stability in conjunction with greater intricacy of mold articles being produced.
The breakthrough of plastics into the field of lenses has added to these demands. For example, plastics are now being used for such critical, but diverse, applications as contact lenses, opthalomological lenses, sunglasses, safety glasses, welders' goggles, camera lenses, rifle scopes, and lenses for industrial optical instruments. Such uses have given rise to a requirement for "lens quality" molds.
In molds for plastic applications there are basically two quality levels: "mold quality" and, the higher or more restrictive level "lens quality". The standard, as used in the art, is one based on the projected capabilities of a given steel. While the test is basically subjective, experience is being gained in part through trial and error. Such experience can now be translated into certain minimum conditions or observations.
Lens quality steel is more restrictive since molds therefrom are prepared to a finer surface finish than mold quality steel. Any imperfection on the polished surface that is visible to the eye will be detrimental. That is, if one can observe pits or marks from holes, inclusions, carbides, etc., the steel is not suitable for lens quality. Likewise, a coarse grain size can produce unevenness. Thus, cleanliness and grain size are just two factors which affect polishability. Others are considered below.
Polishability will improve with increasing hardness. A minimum of R.sub.c 30 is therefore required for a good mold finish. However, if a high lustre is desired, such as for lens quality applications, a hardness of at least R.sub.c 54 is necessary. Adequate hardness is also essential to proper wear resistance. To resist abrasive wear from such plastics as the very demanding thermosetting resins, a hardness of at least R.sub.c 54 is needed.
Finally, there are metallurgical factors which affect polishability. (1) Excessive retained austenite, a relatively soft phase in the harder martensitic matrix, does not take a polish well. That is, polishing of a steel surface containing excessive retained austenite results in a random pattern of hills and valleys, the valleys representing the retained austenite which is softer and is abraded more readily during polishing. (2) Carbides, whether large globular carbides or carbide networks, tend to stand in relief after polishing. As a consequence, the carbides should be fine and uniformly dispersed. (3) Presence of non-martensitic constituents such as bainite, pearlite or ferrite in the microstructure will increase surface roughness since they are softer than martensite.
The die steel of the present invention was developed to meet the above demands. Through research and development, with the above criteria as goals, the present invention resulted in the development of a new die steel having the following properties:
1. High surface hardness, preferably at least 55 HRC, to obtain the desired lustre and high abrasion resistance. PA1 2. Freedom from harmful inclusions and a homogeneous microstructure for optimum polishability and photo etchability. PA1 3. Dimensional stability in heat treatment to minimize clean up. PA1 4. Sufficient toughness at the high hardness level to prevent cracking under the injection pressure load encountered in the plastic molding operations. PA1 Surface Roughness (.mu.-inch)=Boron factor [7.07-12.5 (% C)+0.72 (% Mn)+0.45 (% Si)-2.9 (% Ni)+1.13 (% Ni).sup.2 +0.87 (% Cr)+2.1 (% V)+1.12 (% Mo)+0.84 (% W)+14 (% Nb)], where the calculated Surface Roughness is no greater than 2.65 .mu.-inch. The Boron factor is 1.0 when boron is not present in the above given range, and 1.74 when boron is present. PA1 Surface Roughness (.mu.-inch)=Boron factor [7.07-12.5 (% C)+0.72 (% Mn)+0.45 (% Si)-2.9 (% Ni)+1.13 (% Ni).sup.2 +0.87 (% Cr)+2.1 (% V)+1.12 (% Mo)+0.84 (% W)+14 (% Nb)], where the calculated Surface Roughness is no greater than 2.65 .mu.-inch. The Boron factor is 1.0 when boron is not present in the above given range, and 1.74 when boron is present.
This unique combination of properties for steels of the present invention is accomplished through a careful balancing of the chemistry. Specifically, within the broad chemistry limits, by weight, of carbon 0.3 to 0.8%, and the maximum quantities of manganese 3.0%, phosphorous 0.025%, sulfur 0.025%, silicon 2.0%, nickel 4.0%, cobalt 4.0%, chromium 3.0%, vanadium 1.0%, molybdenum 1.5%, tungsten 1.5%, niobium 0.1%, titanium 0.5%, aluminum 0.1%, optionally boron between 0.0005 and 0.012%, balance iron, the desired properties can be achieved through adherence to the following equation:
Most of the steels used in the plastic industry as molds were developed prior to the recent expansion of such industry. Few, if any of the steels, were developed specifically as mold steels. As a consequence, such presently used steels often exhibit certain features which minimize their usefulness as plastic mold steels. Such features became more obvious as the demands of the plastic industry increased, particularly in the molding of lenses. Four different steels, whose nominal chemistry is listed in Table I, represent typical steel presently used and/or promoted for plastic mold applications.
TABLE I ______________________________________ Prior Art Mold Steels C Mn Si Cr Mo Fe ______________________________________ A .50 .70 .25 3.25 1.40 bal. B .50 1.00 .30 1.10 .25 bal. C .30 .80 .50 1.70 .40 bal. D .35 .25 .50 13.00 bal. ______________________________________
The first of such prior art mold steels, steel A, though possessing a combination of good toughness and wear resistance, suffers the problem known as chemical banding. Many carbon and low alloy steels in the hot-worked condition exhibit banding, defined as a fibrous microstructure of layered pearlite and ferrite. The term banding has also been used to describe other phenomenon where the microstructure had a periodic or intermittent variation of alloy content in a laminated form. The inhomogeneity in alloy steels does not necessarily lead to the production of two separate phases as in carbon steels. Steel A, when treated in the conventional manner reveals bands which are tempered martensite but nevertheless have different chemical composition and micro-hardnesses. Such differences present problems in achieving a highly polished surface.
Another drawback of alloy steels is the presence of complex alloy carbides. Such carbides can appear with large amounts of chromium, vanadium, molybdenum and tungsten. Such carbides may be eliminated through the use of high austenizing temperatures to insure that the carbon is brought into solution. However, with high austenizing temperatures difficulty in controlling dimensions may arise. On the other hand, should the carbides not be put into solution, they will tend to segregate into bands. On the polished surface such segregated bands will appear as carbide streaks. Generally, these carbides are harder than the matrix and will stand out in relief above the matrix. Such a feature presents an obvious problem in attempting to polish a surface containing the carbides to a high lustre.
Alloys B and C are two further alloy steels used for mold applications, but whose chemistries are leaner than the chemistry of Alloy A. An undesirable feature of Alloy B is that it is a low hardenability steel with limited oil hardenability. Also, like its companion Alloy C, the significant amount of Cr and Mo present in the alloy makes it susceptible to segregation. Similarly, Alloy C has its drawbacks which limit its effectiveness as a lens quality mold steel. For instance, Alloy C is a prehardened mold steel characterized by low hardness, i.e. 285/321 HB. This results in a mold surface having insufficient lustre, the liklihood of smeared surface metal, and low wear resistance.
As noted above high hardness is a necessary property of mold steels to achieve a highly polished surface. One of the most serious drawbacks to Alloy D (Type 420 stainless steel) is that it can be hardened to only about 50/52R.sub.c.
The present invention avoids the shortcomings of the prior art steels through the economical use of such alloying agents as tungsten, vanadium, molybdenum, chromium and nickel. However, since a high level of hardenability was required, a certain amount of such alloy additions was required. The present invention recognized the way of achieving the high hardenability while minimizing surface roughness.